Gallium nitride for liquid crystal electrodes

ABSTRACT

Described herein is a liquid crystal (LC) device having Gallium Nitride HEMT electrodes. The Gallium Nitride HEMT electrodes can be grown on a variety of substrates, including but not limited to sapphire, silicon carbide, silicon, fused silica (using a calcium fluoride buffer layer), and spinel. Also described is a structure provided from GaN HEMT grown on large area silicon substrates and transferred to another substrate with appropriate properties for OPA devices. Such substrates include, but are not limited to sapphire, silicon carbide, silicon, fused silica (using a calcium fluoride buffer layer), and spinel. The GaN HEMT structure includes an AlN interlayer for improving the mobility of the structure.

FIELD OF THE INVENTION

The structures and techniques described herein relate to liquid crystaldevices and, more particularly to electrodes used in liquid crystaldevices.

BACKGROUND OF THE INVENTION

As is also known in the art, liquid crystal (LC) devices utilizetransparent electrodes which are necessary to establish electricalfields within the device to produce desired and controllablemodifications to the liquid crystal optical material properties. Onewidely used method of electrode formation to date is the deposition of athin-film semiconductor such as indium-tin-oxide (ITO) or In₂O₃. Suchconductive thin film semiconductors are inherently lossy particularly inthe infrared wavelengths due to their high electron concentrations andlow carrier mobilities.

Transparent conductors are required in various applications. Forexample, in optical phased arrays (OPA's) a steered beam passes throughelectrical conductors that bias and address the liquid crystal moleculesin the device. Preferably, the conductors exhibit very low electricalresistance as well as very low absorption at the wavelength of thesteered beam. Using conventional techniques to provide transparentelectrodes in OPA elements, indium oxide is disposed on sapphiresubstrates. To achieve satisfactory conductivity, the indium oxide ishighly doped. In current technology, nonstoichiometric indium oxide isused as the transparent conductor. For a sheet resistance of 400 ohm/sq,the optical absorption to 1.06 μm laser light is 0.75-1.0%. Loweroptical absorption and resistivity are desired for transparentelectrodes to improve system performance, reduce optical losses, andreduce adverse laser heating in the system. Thus, one drawback to usinghighly doped indium oxide as electrodes (e.g. in OPA elements) is thatsuch electrodes increase optical absorption of a steered optical beam.

Another drawback with the above-described approach is the use ofsubstrates that exhibit birefringence (e.g. sapphire substrates).Substrates exhibiting birefringence alter the polarization of a steeredoptical beam. For polarization sensitive applications, an alternativesubstrate material having low optical absorption and minimal or nobirefringence is desired. Thus, one alternative to using sapphire as asubstrate is to use cubic spinel, (which does not exhibit birefringence)as a substrate. One drawback of crystal cubic spinel, however, is thatit is relatively expensive compared with other substrate materials (e.g.compared with the cost of sapphire, for example). Furthermore crystalcubic spinel substrates are only available in relatively small diameters(e.g. two-inch diameters). For some applications, only one-half of anOPA can be fabricated from one two-inch substrate.

SUMMARY OF THE INVENTION

To overcome the above-noted drawback of using highly doped indium oxideconductors, and in accordance with the concepts, structures andtechniques described herein, highly conductive gallium nitride highelectron mobility transistor structures (GaN HEMTs) are deposited orotherwise disposed on a substrate material, the structure having amobility that is higher than the mobility of indium oxide. Consequently,fewer carriers are needed to achieve the same conductivity achievedusing the prior art indium oxide approach. This results in less freecarrier absorption. Thus, the use of GaN HEMTs and spinel overcome theabove-noted drawbacks with respect to optical absorption of a steeredoptical beam and birefringence which alters polarization of a steeredoptical beam.

To provide a relatively inexpensive structure while overcoming theabove-noted drawback of using substrates (e.g. sapphire substrates) thatexhibit birefringence or using expensive substrates (e.g. cubic spinel)which do not exhibit birefringence, in one embodiment, a GaN HEMT isdeposited on large area, relatively inexpensive, single crystal siliconsubstrates. The GaN HEMT is then transferred to a substrate which issubstantially optically transparent at wavelengths of interest andhaving little or no birefringence and having a cost which is less thanthe cost of single crystal spinel. Exemplary symmetric crystals suitablefor such use include, but are not limited to fused silica, poly spineland zinc sulfide (ZnS).

With this particular arrangement, a GaN HEMT having improved mobility isprovided. The use of an aluminum nitride (AlN) interlayer results in theHEMT having increased conductivity from improved mobility since alloyscattering at the AlGaN/GaN interface is reduced by the insertion of theAlN interlayer. Furthermore, the deposition of the GaN HEMT on largerarea relatively inexpensive silicon single crystal substrates with asubsequent transfer to an optically suitable substrate (e.g. a substratewhich may be inexpensive and/or a non-birefringent material) separatesout the material growth, which requires high quality single crystalsubstrates of compatible crystal structure, from the final mountingsubstrate which no longer needs to be even single crystal. Also, the useof large area wafers enables many more OPA elements to be fabricated andthus fewer wafers are needed than in prior art approaches.

In accordance with a further aspect of the concepts, structures andtechniques described herein, an optical window structure includes asubstrate and transparent conductive electrodes provided from galliumnitride (GaN) HEMT. The use of a GaN HEMT as a transparent conductiveelectrode material results in low optical losses due to the highmobility of the carriers (in the range of 1600-2000 cm2/V-s) allowinglower carrier densities to be used for same required conductivity. A GaNHEMT is also transparent from the visible through the near infraredwavelengths.

A gallium nitride HEMT is commonly grown on a variety of substrates,including but not limited to sapphire, silicon carbide, silicon, andeven fused silica (using a calcium fluoride buffer layer). For thepurpose of liquid crystal (LC) applications, a substrate is selectedwhich is transparent to the wavelength of operation of the LC device(e.g. an optical phased array, an adaptive optic, or a polarizationcorrector). In one embodiment a GaN layer is epitaxially grown to aspecified thickness, for example 1 micron (um). Growth of a second layerof higher bandgap aluminum gallium nitride (Al_(x)Ga_(1-x)N) where x isbetween 0 and 1 or aluminum nitride (AlN) forms a two-dimensionalelectron gas at the AlGaN/GaN interface, from which the conductivity isderived. Photolithographic processing can be used to define electrodestructures on a surface of a wafer. Etching the second layer (e.g. theAl_(x)Ga_(1-x)N layer) provides semi-insulating regions between thedesired electrode patterns where this second layer has been removed.Subtractive or additive processes (or a combination of subtractive andadditive processes) may be used to provide the desired structure. In oneembodiment, an AlN interlayer is disposed between a GaN layer and anAlGaN top layer. With this particular arrangement, a structure havingimproved mobility provided by inclusion of the AlN interlayer isprovided.

In one embodiment, an LC phase retarder is provided from GaN HEMT layersgrown on sapphire substrates. A single electrode of AlGaN/GaN isprovided on each containing surface.

In accordance with a further aspect of the present invention, low cost,large area transparent electrode-substrate combinations that do notexhibit birefringence and have improved conductivity are described aswell as a process for providing such structures.

A process for providing transparent electrode-substrate combinationsthat do not exhibit birefringence and have improved conductivityincludes: (a) growing a GaN HEMT structure containing an aluminumnitride (AlN) interlayer on a large area, silicon substrate having acrystallographic orientation which promotes growth of hexagonal AlN andGaN and which may be inexpensive; (b) after growth, an AlGaN surface ofthe GaN HEMT structure is mounted to a carrier wafer; (c) the GaN HEMTstructure containing the AlN interlayer is then removed or otherwiseseparated from the silicon substrate; (d) an optically suitablesubstrate (which can have cubic symmetry or be amorphous orpolycrystalline) is then bonded or otherwise secured to the exposedsurface (in some applications it may be preferred to utilize anoptically suitable substrate having little or no birefringence); (e) thecarrier wafer is then removed or otherwise separated from the AlGaNsurface; and (f) electrode(s) and ohmic contacts(s) are then provided onthe AlGaN surface (e.g. via a patterning technique or any othertechnique well-known to those of ordinary skill in the art).

The benefits of this structure and process over conventional structuresand processes are: (a) the structure and process described herein isrelatively inexpensive (compared to existing prior art approaches) anduses large area silicon substrates (e.g. relatively inexpensive siliconsubstrates with the desired <111> orientation can be obtained indiameters of 200-mm); (b) the GaN HEMT structure exhibits improvedconductivity compared to ITO due to the improved transport properties ofthe GaN HEMT which are further enhanced by the AlN interlayer; and (c)the GaN HEMT has low optical absorption and in particular, lower thanITO.

In one embodiment, a silicon substrate having a <111> crystallographicorientation is used due to the hexagonal net of silicon atoms whichpromotes growth of hexagonal AlN and GaN. The silicon substrate may beremoved by either a dry or wet etch. This process is readily achieveddue to the much higher chemical reactivity of silicon compared to AlNand GaN.

The substrate may be bonded to the etched surface with an epoxy or anadhesive (e.g. a UV curing adhesive) or any other material capable ofsecuring together the substrate and etched surface and which has a lowNIR (near infrared) absorption (e.g. an absorption lower than that ofITO such as about 0.3% or less) and which is compatible with LCapplications (e.g. OPAs, adaptive optics, polarization correctors, LCphase retarders). Substrates suitable for use in LC applications(including those listed above), include, but are not limited topolycrystalline spinel substrates, ALON substrates, fused silicasubstrates, and zinc sulfide (ZnS) substrates.

U.S. Pat. No. 6,099,970, issued to Bruno et al, and assigned to TRW, IncRedondo Beach, Calif., describes using a plurality of AlGaN/GaN/AlGaNquantum wells for transparent electrodes. The quantum wells increase thecarrier concentration and thereby lower the conductivity but alsoincrease the free carrier absorption.

It should be appreciated, however, that the structure described herein,is a single sided AlGaN/AlN/GaN heterojunction. The conductivity, whichis inversely proportional to mobility, is increased by increasing themobility with the AlN interlayer. The electron mobility is increased byinserting an AlN interlayer from approximately 1600 to 2000 cm²Vs.Birefringence is reduced (or in some cases minimized or even eliminated)by bonding the layer stack to a material with low or no birefringence(e.g. a cubic, amorphous, or polycrystalline structure). One importantfeature of this process is that the crystal growth process, whichrequires high quality single crystal material of compatible crystallinestructure, is not dependent upon the choice of final substrate material.This characteristic permits use of appropriate polycrystalline materialsof various lattice constants and orientations. For example,polycrystalline spinel can be used as the final substrate material whichis less expensive and available in larger areas than single crystalspinel.

In accordance with a further aspect of the present invention, describedherein is a structure and process that reduces cost of optical phasedarray elements, reduces (or in some cases minimizes) polarizationchanges in a steered optical beam, reduces (or in some cases minimizes)optical absorption of a steered optical beam, and increases conductivityof transparent electrodes.

In accordance with a still further aspect, a liquid crystal (LC) devicehaving gallium nitride (GaN) HEMT electrodes is described. The GaN HEMTelectrodes can be grown on a variety of substrates, including but notlimited to sapphire, silicon carbide, silicon, fused silica (using acalcium fluoride buffer layer), and spinel. In LC applications, thesubstrate is selected from a material which is transparent to thewavelength of operation of the LC device (e.g. an optical phased array,an adaptive optic, or a polarization corrector). In one embodiment, theGaN electrode layer is epitaxially grown to a specified thickness.Growth of a second layer of higher bandgap Al_(x)Ga_(1-x)N or AlN formsa two-dimensional electron gas, from which the conductivity is derived.In one embodiment, photolithographic processing (using either positiveor negative masks) can be used to define the electrode structures on theface of the wafer. An additive or subtractive process (e.g. etching thesecond AlGaN or AlN layer) can be used to provides semi-insulatingregions between the desired electrode patterns where the second layer isabsent (e.g. removed via an etching process).

Also described is a structure provided from a GaN HEMT grown on a largearea silicon substrate and transferred to a second, different substratewith appropriate properties for OPA devices. In one embodiment, an AlNinterlayer is disposed between a GaN layer and an AlGaN top layer. Withthis particular arrangement, a GaN HEMT structure having improvedmobility provided by inclusion of the AlN interlayer is provided.Materials suitable for use as the second substrate include, but are notlimited to materials such as poly spinel, or fused silica (using acalcium fluoride buffer layer). Poly SiC as well as Zinc Sulfide (ZnS)may also be used as materials for the second substrate. Depending uponthe needs/requirements of a particular application (e.g. costcharacteristics, thermal conductivity characteristics, spectral bandcharacteristics, birefringence characteristics), any of the followingmaterials may be used:

Thermal Special Material Cost Conductivity Band Birefringent FusedSilica Low Low vis-NIR No AlON Med High vis-NIR No CaF2 Med HighUV-vis-NIR No GaAs Med Very high NIR-LWIR No GGG (Gd3Ga5O12) HighMed-High vis-NIR No Sapphire Med High vis-NIR Yes Silicon Carbide HighVery high NIR Yes polycrystalline Spinel Med High vis-NIR No Y₂O₃ HighHigh vis-NIR No YAG (Y₃Al₁₅O₁₂) High High vis-NIR No Zinc Selenide HighHigh NIR-LWIR No Zinc Sulfide (ZnS) Med High vis-NIR No polycrystallineSiC Med High NIR Yes

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the concepts, structures and techniquesdescribed herein may be more fully understood from the followingdescription of the drawings in which:

FIG. 1 is a perspective view of a transparent window using a galliumnitride (GaN) high electron mobility transistor (HEMT) structure; and

FIG. 2 is a perspective view of a transparent multi-electrode windowusing a GaN HEMT structure;

FIG. 3 is a cross-sectional view of a GaN HEMT structure having analuminum nitride (AlN) interlayer grown on a silicon substrate; and

FIG. 3A is a cross-sectional view of a GaN HEMT structure having an AlNinterlayer grown on a transparent, low birefringence substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 a transparent multi-electrode window using agallium nitride (GaN) high electron mobility transistor (HEMT) structureincludes a substrate 12 having an aluminum nitride (AlN) layer 14disposed thereover with a semi-insulating gallium nitride (GaN) layer 16disposed thereover. Substrate 12 may be provided from a number ofmaterials, including but not limited to sapphire, SiC, Si, or Spinel. Inone exemplary embodiment, GaN layer 16 is provided having a thicknesstypically in the range of about 1-3 microns (μm) and AlN layer 14 isprovided having a thickness typically less than about 0.1 μm. In someapplications, a thickness of 0.05 μm is preferred.

Disposed over the semi-insulating GaN layer 16 is a topmost AlN orAl_(x)Ga_(1-x)N layer 18. In one embodiment, the topmost Al_(x)Ga_(1-x)Nlayer 18 is provided having a thickness typically in the range of about50-400 angstroms (Å) with about 200 Å being preferred in someapplications.

Referring now to FIG. 2, in which like elements of FIG. 1 are providedhaving like reference designations, topmost layer 18 is provided as aplurality of individual electrodes 20. In one embodiment, the pluralityof individual AlN or Al_(x)Ga_(1-x)N electrodes 20 are provided via amicroelectronic patterning process and x is in the range of about 0.1 toabout 0.4 with about 0.25 being typical. In other embodiments, valuesoutside that range may also be used. A person of ordinary skill in theart will appreciate how to select a value for x for a particularapplication. It should, of course, be appreciated that other techniquesother than microelectronic patterning may also be used to provideelectrodes 20.

In one embodiment, the microelectronic patterning process includesspinning and baking of a photoresist over the topmost layer 18. Next,the photoresist is exposed using a mask aligner, stepper, electron beampatterning system, or any other technique known to those of skill in theart. The photoresist is then developed exposing regions between desiredconductive regions 20.

The top layer 18 may be etched to provide electrodes 20. In oneembodiment, the top layer may be etched by reactive ion etching in achlorine gas. Other etching technique may, of course, also be used. Thephotoresist is then removed leaving patterned conductive electrodes 20as shown in FIG. 2.

Referring now to FIGS. 3 and 3A in which like elements are providedhaving like reference designations, a structure and process aredescribed to provide low cost, large area transparentelectrode-substrate combinations that do not exhibit birefringence andthat have improved conductivity. A GaN HEMT structure 30 includes asubstrate 12 provided from <111> silicon (Si) having an AlN buffer layer14 disposed thereover. Disposed over layer 14 is a GaN buffer layer 16.Disposed over the GaN buffer layer 16 is an AlN interlayer 20 anddisposed over interlayer 20 is an AlGaN barrier layer 18.

It should be appreciated that AlN buffer layer 14 is grown on siliconsubstrate 12 and provides insulation between GaN buffer layer 16 andsubstrate 12. AlN layer 20, however, is an interlayer between AlGaN toplayer 18 and GaN buffer layer 16. AlN layer 20 (also referred to asinterlayer 20) improves the mobility and increases carrier density ofthe structure 30 and consequently increases the conductivity of the GaNHEMT structure 30 for a given free carrier density. The GaN HEMTexhibits improved conductivity due to interlayer 20. In someapplications it is desirable to keep the interlayer thin (e.g. on theorder of 10 Å) since a relatively thick interlayer increases thedifficulty with which ohmic contacts can be made to the structure. Itshould, however, be appreciated that the interlayer thickness to use inany particular application depends, at least in part, upon thesmoothness of the underlying surface (i.e. the surface over which theinterlayer is disposed). For example, in some applications, if theunderlying surface has a smoothness in the range of 5 Å-15 Å, then aninterlayer having a thickness of about 10 Å may be used.

The process comprises growing a GaN HEMT structure containing an AlNinterlayer on a large area, relatively inexpensive silicon <111>substrate. This substrate orientation is used due to the hexagonal netof silicon atoms which promotes growth of hexagonal AlN and GaN.

After growth, the AlGaN layer 18 is temporarily mounted to a carrierwafer (not shown). The silicon substrate is removed by either a dry orwet etch. This process is readily achieved due to the much higherchemical reactivity of silicon compared to AlN and GaN. An opticallysuitable substrate 22 is bonded or otherwise secured to the etchedsurface. This may be accomplished, for example, using an epoxy or anadhesive (e.g. such as a UV curing adhesive) or using any othertechnique known to those of ordinary skill in the art. If an epoxy oradhesive is used, it preferably should have low NIR absorption and becompatible with an OPA configuration.

Substrates suitable for bonding to the etched surface are shown in Table1.

TABLE 1 Thermal Special Material Cost Conductivity Band BirefringentFused Silica Low Low vis-NIR No AlON Med High vis-NIR No CaF2 Med HighUV-vis-NIR No GaAs Med Very high NIR-LWIR No GGG (Gd3Ga5O12) HighMed-High vis-NIR No Sapphire Med High vis-NIR Yes Silicon Carbide HighVery high NIR Yes Polycrystalline Spinel High High vis-NIR No Y₂O₃ HighHigh vis-NIR No YAG (Y₃Al₁₅O₁₂) High High vis-NIR No Zinc Selenide HighHigh NIR-LWIR No Zinc Sulphide Med High vis-NIR No polycrystalline SiCMed High NIR Yes

The carrier wafer is then removed from the AlGaN surface. One or moreelectrodes and/or ohmic contacts may then be provided (e.g. using apatterning technique or any additive and/or subtractive process) on theAlGaN surface to provide the structure having one or more electrodes andohmic contacts. It is believed that the general process of transferringa GaN HEMT from a large area single crystal silicon <111> substrateneeded for single crystal growth to another substrate (not necessarilybeing single crystal) but having the proper optical properties for OPA'sis a novel technique for providing optical windows suitable for use in awide variety of optical devices and applications.

The benefits of this structure and process over conventional structuresand processes include but are not limited to: (a) the use of inexpensiveand large area silicon substrates; and (b) the use of an AlN interlayerstructure with a GaN HEMT structure (the GaN HEMT exhibits improvedconductivity due to the AlN interlayer).

The structures in FIGS. 3 and 3A use a single sided AlGaN/AlN/GaNheterojunction. The conductivity is proportional to mobility (whileresistivity is inversely proportional to mobility) and is increased byincreasing the mobility with the AlN interlayer. It has been discoveredthat when growing GaN HEMTs on SiC, the electron mobility is increasedby inserting an AlN interlayer from approximately 1600 to 2000 cm²/Vs.Also, birefringence is reduced (or in some cases minimized or eveneliminated) by bonding (or otherwise securing together) the layer stackto a material with low or no birefringence (of cubic, amorphous, orpolycrystalline structure). One important feature of this process isthat the crystal growth process, which requires high quality singlecrystal material of compatible crystalline structure, is not dependenton the choice of final substrate material. This characteristic permitsappropriate polycrystalline materials of various lattice constants andorientations. For example, polycrystalline spinel can be used which ischeaper and available in larger areas than single crystal spinel.

Having described preferred embodiments which serve to illustrate variousconcepts, structures and techniques which are the subject of thispatent, it will now become apparent to those of ordinary skill in theart that other embodiments incorporating these concepts, structures andtechniques may be used. Accordingly, it is submitted that the scope ofthe patent should not be limited to the described embodiments but rathershould be limited only by the spirit and scope of the following claims.

1. An optically transparent electrode comprising: a substrate ofoptically-transparent material, said substrate having a region on asurface thereof corresponding to a Gallium Nitride (GaN) regioncomprising less than the entire area of said surface of said substratesheet, wherein said substrate exhibits substantially uniformtransmission characteristics over said surface to radiant energy of apredetermined wavelength; and a plurality of electrodes disposed on thesurface of said GaN layer and electrically isolated from each other by asemi-insulating region and, each of said plurality of individualelectrodes comprised of one of AlN or Al_(x)Ga_(1-x)N.
 2. The electrodeaccording to claim 1 wherein said substrate of optically-transparentmaterial corresponds to a substrate of optically-transparent, insulatingmaterial.
 3. The electrode according to claim 1 wherein said substrateof optically-transparent material corresponds to one of: Fused Silica,AlON, CaF2, GaAs, GGG (Gd3Ga5O12), sapphire, silicon carbide,polycrystalline SiC, spinel, polycrystalline spinel, Y₂O₃, YAG(Y₃Al₁₅O₁₂), Zinc Selenide and Zinc Sulphide.
 4. The electrode accordingto claim 1 wherein said substrate is transparent to the wavelength ofoperation of a liquid crystal device.
 5. The electrode according toclaim 4 wherein the liquid crystal device corresponds to one of: anoptical phased array, an adaptive optic, or a polarization adjustingelement.
 6. The electrode according to claim 1 wherein the GaN layer isepitaxially grown to a specified thickness.
 7. The electrode accordingto claim 6 further comprising a second epitaxially grown layer of higherbandgap than the first layer.
 8. The electrode according to claim 6further comprising an Al_(x)Ga_(1-x)N or an AlN layer disposed over saidGaN layer, wherein said Al_(x)Ga_(1-x)N or AlN layer forms atwo-dimensional electron gas, from which the conductivity is derived 9.The electrode according to claim 8 wherein photolithography defines theelectrode structures on the face of the wafer.
 10. The electrodeaccording to claim 9 wherein etching the Al_(x)Ga_(1-x)N layer providessemi-insulating regions between the desired electrode patterns wherethis second layer has been removed.
 11. The electrode according to claim1 wherein said predetermined wavelength corresponds to a frequency inthe infrared band.
 12. A method for providing transparentelectrode-substrate combinations, the method comprising: (a) growing aGaN HEMT structure containing an AlN interlayer on a silicon substratehaving an orientation selected to promote growth of hexagonal AlN andGaN; (b) disposing an AlGaN surface of the GaN HEMT structure on asurface of a carrier wafer; (c) separating the GaN HEMT containing anAlN interlayer from the silicon substrate to expose a surface of the GaNHEMT; (d) securing a substrate possessing little or no birefringence tothe exposed surface of the GaN HEMT; (e) separating the carrier waferfrom the AlGaN surface; and (f) providing one or more electrodes andohmic contacts on the AlGaN surface.
 13. The method of claim 12 whereinsecuring a substrate to the exposed surface of the GaN HEMT comprisesbonding a substrate to the exposed surface with one of: an epoxy; anadhesive; or a UV curing adhesive.
 14. The method of claim 13 whereinthe epoxy, adhesive or UV curing adhesive is provided having low nearinfrared (NIR) absorption and is compatible with an LC configuration.15. The method of claim 14 wherein bonding a substrate to the exposedsurface of the GaN HEMT comprises one of: (a) bonding a polycrystallinespinel substrate to the exposed surface of the GaN HEMT; (b) bonding anALON substrate to the exposed surface of the GaN HEMT; (c) bonding afused silica substrate to the exposed surface of the GaN HEMT; or (d)bonding a ZnS substrate to the exposed surface of the GaN HEMT.
 16. Themethod of claim 15 wherein separating the GaN HEMT with an AlNinterlayer from the silicon substrate comprises separating the GaN HEMTand AlN interlayer structure from the silicon substrate by one of: a dryetch technique; or a wet etch technique.
 17. The method of claim 16wherein providing one or more electrodes and ohmic contacts on the AlGaNsurface comprises patterning one or more electrodes and ohmic contactson the AlGaN surface.
 18. The method of claim 12 wherein growing a GaNHEMT structure containing an AlN interlayer on a silicon substratehaving an orientation selected to promote growth of hexagonal AlN andGaN comprises growing a GaN HEMT structure containing an AlN interlayeron a silicon substrate having a <111> crystallographic orientation. 19.The method of claim 12 wherein bonding a substrate to the exposedsurface of the GaN HEMT comprises bonding a substrate having high cubicsymmetry and little or no birefringence to the exposed surface of theGaN HEMT.
 20. A gallium nitride (GaN) high electron mobility transistor(HEMT) structure comprising: (a) a substrate having first and secondopposing surfaces, said substrate being optically transparent at awavelength of interest; (b) an AlN buffer layer disposed over a firstsurface of said substrate; (c) a GaN buffer layer disposed over a firstsurface of said AlN buffer layer; (d) an AlN interlayer disposed over afirst surface of said GaN buffer layer; and (e) an AlGaN barrier layerdisposed over said AlN interlayer.
 21. The GaN HEMT structure of claim20 wherein said substrate is provided as one of: a polycrystallinespinel substrate; an ALON substrate; a fused silica substrate; and anZnS substrate, a CaF2 substrate, a GaAs substrate, a GGG (Gd3Ga5O12)substrate, a sapphire substrate, a silicon carbide substrate, apolycrystalline SiC substrate, a Y₂O₃ substrate, an YAG (Y₃Al₁₅O₁₂)substrate, a Zinc Selenide substrate and a Zinc Sulphide substrate.